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Avalanche Zoning

Avalanche Zoning is a process for identifying avalanche-prone areas on a map and regulating the types of land
uses allowed based on the level of risk and severity of consequences of avalanches. The level of
acceptable risk varies depending on the type of land use and local,
regional and national attitudes towards individual choice and government's responsibility to prevent disasters
and the responses that follow.

Avalanche zoning in North America is based on concepts from Switzerland. The Swiss defined three categories of avalanche zoning
in the 1950s following a severe avalanche winter of 1950-51. In Switerland, the "Red" or "High" Hazard Zone is defined as an
area with either frequent avalanches (average return period of 30 years or less) or areas
where reference impact pressures are ~600 psf or greater. Occupied structures are generally prohibited in the "Red" zone.

A "Blue" or "Moderate" hazard zone was defined as an area where avalanches are less frequent (more than 30 year average return period) and
where the longer return period avalanches (up to 300 years average return period) impart pressures of less than 600 psf on a large flat rigid
plate normal to the direction of flow. Occupied structures are permitted in "Blue" zones only if the structures are designed to
withstand the anticipated avalanche loads. Critical structures such as hospitals, schools and emergency response facilities are
not permitted within "Blue" zones.

A "Yellow" (very low hazard) Zone is sometimes used to define an area beyond the Blue Zone in which
avalanches which are very rare (average return periods of more than 300 years), or areas subject to the
blast or low-density snow or air from powder avalanches with dynamic pressures of 0.3 tons/m2
(62 lbs/ft2). Land use is not restricted in the Yellow Zone, but design consideration to potential
avalanche loads on structures is advisable.

In the United States, there are no national standards. Local jurisdictions have standards that fit local political and economic
conditions (Ketchum, ID, Pitkin County CO, Juneau, AK).